Motion of charged particles is often controlled by wire gates, as employed in applications such as electron microscopy, mass spectrometry, and ion mobility spectrometry. Electric fields can be generated by applying electric potentials to the wires, and these electric fields can act on charged particles to alter their motion. Many different kinds of wire gates have been considered in the art for controlling charged particle motion. One kind of gate commonly known as a Bradbury-Nielson gate (BNG) can provide excellent performance, especially in demanding applications requiring precise timing control, such as Hadamard transform time-of-flight mass spectrometry.
A BNG includes a set of evenly spaced, co-planar, and parallel wires. The wires alternate in a repeating ABAB pattern, where all of the A wires are electrically connected to each other, all of the B wires are electrically connected to each other, and the set of A wires is electrically isolated from the set of B wires. The main advantage of the BNG is that its electric field decays very rapidly as distance increases away from the plane of the wires. The deflection region, where electric fields are non-negligible, extends out to about one wire spacing from the plane of the BNG. Thus decreasing the BNG wire spacing decreases the size of the deflection region, which in turn improves the time resolution of the BNG.
Although fabrication of BNGs having large wire spacing tends to be straightforward, BNG fabrication difficulty increases significantly as the wire spacing decreases. The main difficulties encountered are precisely placing the wires of the BNG (i.e., so they are parallel, co-planar and evenly spaced with the desired spacing), and providing alternating electrical contact to the BNG wires as described above. These fabrication difficulties are further increased by the common requirement in practice that the BNG have a large active area (i.e., on the order of 5 cm×5 cm).
Several methods have been considered in the art to address some of these issues. In an article by Vlasek et al. (Rev. Sci. Instrum., 67(1), pp. 68-72, January 1996) a wire spacing of 1 mm was achieved by weaving a wire through holes drilled through two frames separated by two threaded rods. An article by Stoermer et al. (Rev. Sci. Instrum., 69(4), pp. 1661-1664, April 1998) demonstrated a wire spacing of 0.5 mm by winding wire on the threads of two nylon screws. A wire spacing of about 0.16 mm is reported by Brock et al. (Rev. Sci. Instrum., 71(3), pp. 1306-1318, March 2000), where wire segments are individually soldered to electrode pads on the BNG frame. The wire positioning and soldering in this case entailed time-consuming manual assembly under a microscope.
A template based approach for BNG fabrication was considered by Kimmel et al. (Rev. Sci. Instrum., 72(12), pp. 4354-4357, December 2001, and in U.S. Pat. No. 6,664,545). In this work, 0.1 mm spaced V-grooves are machined into a plastic mount, and then two sets of wires are wrapped into the grooves under a microscope. Although this approach reduces fabrication time compared to the approach of Brock et al., it still entails lengthy microscope assembly work.
Microfabrication methods have also been employed for BNG fabrication. Examples in the art of such methods include U.S. Pat. No. 6,977,381, US Patent Application 2005/0258514, and US Patent Application 2006/0231751. Although microfabrication methods can provide BNGs having very small wire spacing (e.g., as low as 0.015 mm), it is difficult for microfabrication methods to provide BNGs having a large active area. For example, in one report of a microfabricated BNG, the maximum active area was on the order of 5 mm by 5 mm.
Accordingly, it would be an advance in the art to provide a BNG fabrication method for large-area BNGs having small wire spacing that does not require laborious assembly under a microscope.